The following U.S. Patent Publications are believed to represent the current state of the art:
U.S. Pat. Nos. 6,648,496; 7,021,809; 7,099,553; 6,972,401; 5,257,340; 6,783,269; 5,905,583; 5,295,047; 7,163,326; and U.S. Patent Applications 2005-0248540; and 2005-0128190